This paper examines the effect that aligned long fiber reinforcement has on the processing characteristics of thermoplastic composites. More specifically, the influence of fiber volume fraction on the transverse shear viscosity of unidirectional composites is explored. Through both experimental evaluation and theoretical modeling, the study focuses on the flow behavior of a model material consisting of unidirectional nylon or glass fibers aligned in a clay matrix. The flow behavior of a commercially produced material (APC-2) composed of unidirectional carbon fibers aligned in a thermoplastic polyether ether ketone (PEEK) matrix is also examined. An experimental technique has been employed that characterizes both the bulk transverse shearing viscosity and the fluid mechanics of such highly filled fiber-resin systems in squeeze flow. Squeeze how experiments were performed for the model material containing various fiber volume fractions and, with specially designed hot platens, for the carbon fiber-PEEK composites. Flow visualization techniques have been developed to measure the velocity profile of the material during flow. A cell model is proposed to calculate the effect of fiber volume fraction on the transverse shear viscosity of aligned fiber composites and, hence, the squeeze force requirements of such materials. The cell model, which calculates individual fiber interactions based on the lubrication approach with a viscous Newtonian or Carreau fluid, demonstrates the effects of varying the fiber volume fraction, fiber diameter and the shear thinning nature of the matrix fluid on the force requirement under constant squeeze rates. Comparisons are made between the experimentally measured squeeze force and the sell model predictions. Good agreement is found at high fiber volume fractions as the lubrication flow assumptions become more accurate.